1. Field of the Invention
The present invention generally relates to a sensor including a lead frame and a method of forming a sensor including the lead frame.
Priority is claimed on Japanese Patent Application No. 2005-50879, filed Feb. 25, 2005, and Japanese Patent Application No. 2005-93636, filed Mar. 29, 2005, the contents of which are incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
In recent years, there has been an increasing requirement for integrating terminal devices such as mobile phones with a GPS (Global Positioning System) function, which indicates information about a user's position. The GPS function can be realized by using a sensor for sensing or measuring a physical quantity such as a magnetic sensor or an acceleration sensor. The magnetic sensor integrated in the terminal device senses or measures a geomagnetic field in order to sense or measure the azimuth or direction in a three-dimensional space of the terminal device. The acceleration sensor integrated in the terminal device senses or measures the direction of motion of the terminal device in order to sense or measure the position of the terminal device. A typical example of the sensor for sensing a physical quantity may include, but is not limited to, a plurality of sensor chips such as magnetic sensor chips that are placed to be tilted from each other. This placement of the sensor chips can be effective to scale down the sensor and reduce the thickness of the sensor.
The sensor with the tilted or sloped sensor chips has a high sensitivity in axial directions vertical to planes that include the sloped sensor chips and a poor sensitivity in other axial directions such as axial directions parallel to a surface of a substrate. This type of sensor will be the main trend in the future.
FIG. 24 is a plan view illustrating a conventional sensor for sensing a physical quantity. FIG. 25 is a cross sectional elevation view of the sensor of FIG. 24. FIG. 26 is a plan view of a lead frame to be used for forming the sensor of FIG. 24. A sensor 1 for sensing a physical quantity includes a pair of sensor chips 2 and 3 that are tilted from each other so as to sense or measure the direction and the magnitude of an external magnetic field. The sensor 1 of FIG. 24 can be formed using a lead frame 4 of FIG. 26. The lead frame 4 can be formed through a press working and/or etching process for a metal plate.
The lead frame 4 of FIG. 26 includes a frame 5 and a pair of stages 6 and 7 that are supported by the frame 5. The frame 5 further includes a rectangle frame portion 5a, a plurality of leads 5b, and modified connection leads 5d. The rectangle frame portion 5a defines a rectangle internal region. The rectangle frame portion 5a has four corners 5c, two long sides and two short sides. The plurality of leads 5b extend from the rectangle frame portion 5a into the internal region. The modified connection leads 5d extend from the corners 5c of the rectangle frame portion 5a into the internal region. The stages 6 and 7 are disposed in the internal region and supported by first and second pairs of the modified connection leads 5d. 
The lead frame 4 has a first center line parallel to the two long sides of the rectangle frame portion 5a and a second center line parallel to the two short sides of the rectangle frame portion 5a. The first and second center lines are perpendicular to each other. The stages 6 and 7 have a rectangle shape. The second center line divides the internal region into two sub-regions. The stages 6 and 7 are disposed in the two sub-regions, receptively, so as to be distanced from the second center line. The stages 6 and 7 are positioned symmetrically with reference to a reflection-symmetric axis of the second center line. Each of the stages 6 and 7 extends symmetrically with reference to another reflection-symmetric axis of the first center line. The frame 5 is symmetrical with reference to the reflection-symmetric axes of the first and second center lines. The stage 6 has a first surface on which a sensor chip 2 is mounted and a second surface opposite to the first surface. The stage 7 has a first surface on which a sensor chip 3 is mounted and a second surface opposite to the first surface. The stage 6 has a pair of projecting parts 8 that project toward the counterpart stage 7 and tilt from a plane including the stage 6 toward a space to which the second surface faces. The projecting parts 8 are distanced from the second center line. The stage 7 has a pair of projecting parts 9 that project toward the counterpart stage 6 and tilt from a plane including the stage 7 toward the space to which the second surface faces. The projecting parts 9 are distanced from the second center line.
The modified connection leads 5d comprise suspension leads that suspend the stages 6 and 7 from the rectangle frame portion 5a. Each of the modified connection leads 5d has a twistable portion 5e that is connected with and adjacent to a side edge 6b or 7b of the stage 6 or 7. The twistable portion 5e comprises a narrow portion that is defined by recessed sides. Twisting the twistable portions 5e tilts or slopes the stage 6 or 7 from a plane including the frame 5.
The sensor 1 shown in FIGS. 24 and 25 includes the plurality of leads 5b, the modified connection leads 5d with the twisted portions 5e, the sloped stages 6 and 7 with the sloped sensor chips 2 and 3, wirings 10 that electrically connect the sloped sensor chips 2 and 3 to the leads 5b, and a resin mold 11 that encapsulates the sensor chips 2 and 3 and the plurality of leads 5b. The resin mold 11 is formed by an injection molding process. When the injection molding process has been completed, the leads 5b and the modified connection leads 5d have outside portions that are positioned outside the resin mold 11 and the rectangle frame portion 5a. The leads 5b and the modified connection leads 5d are detruncated to remove the outside portions and the rectangle frame portion 5a, thereby completing the sensor 1. This is disclosed in Japanese Unexamined Patent Application, First Publication, No. 2004-128473.
A broken line in FIGS. 24, 25 and 26 represents the periphery of the resin mold 11. The resin mold 11 has a rectangle shape in plan view and a trapezoidal shape in cross section. The resin mold 11 has a first surface and a second surface 11a that is opposite to the first surface. The first surface is a top surface and the second surface 11a is a bottom surface. The lead frame 4 has a first surface and a second surface 4a opposite to the first surface. The lead frame 5 has a first surface and a second surface 5g that is opposite to the first surface. The second surface 4a includes the second surface 5g. The second surface 5g of the lead frame 5 appears in the second surface 11a of the resin mold 11. Namely, the second surface 5g is exposed from the second surface 11a. The projecting parts 8 and 9 have top portions 8a and 9a that are positioned on the second surface 11a of the resin mold 11. The stages 6 and 7 with the sensor chips 2 and 3 are sloped from the second surface 11a of the resin mold 11. The stage 6 extends in a first plane that tilts from a second plane that includes the second surface 11a of the resin mold 11. The stage 7 extends in a third plane that tilts from the second plane and also from the first plane. The first and third planes cross each other at an acute angle. The sensor chips 2 and 3 are mounted on the sloped stages 6 and 7. The sloped stages 6 and 7 with the sloped sensor chips 2 and 3 are fixed to and encapsulated by the resin mold 11.
A conventional method of forming the sensor 1 will be described. FIG. 27A is fragmentary cross sectional elevation view of the lead frame 4 of FIG. 26 in a step involved in a method of forming the sensor 1 of FIGS. 24 and 25. FIG. 27B is a fragmentary cross sectional elevation view of the lead frame 4 in another step involved in the method of forming the sensor 1 of FIGS. 24 and 25. FIG. 27C is a fragmentary cross sectional elevation view of the lead frame 4 in still another step involved in the method of forming the sensor 1 of FIGS. 24 and 25.
As shown in FIGS. 26 and 27A, a metal plate having a predetermined inner region is prepared. The predetermined inner region of the metal plate is subjected to a photo-etching process to reduce the thickness in half and to form a thickness-reduced plate. The thickness-reduced plate is then subjected to a press working and/or etching process to form the lead frame 4 that includes the frame 5 and the stages 6 and 7 with the projecting parts 8 and 9. The stages 6 and 7 are supported by the modified connection leads 5d that are connected to the rectangle frame portion 5a. Each of the modified connection leads 5d has the twistable portion 5e that comprises the narrow portion defined by the recessed sides. The projecting parts 8 extend from the stage 6 toward the counterpart stage 7 and tilt from the plane including the stage 6. The projecting parts 9 extend from the stage 7 toward the counterpart stage 6 and tilt from the plane including the stage 7.
The sensor chips 2 and 3 are bonded onto the stages 6 and 7, respectively. The sensor chips 2 and 3 are electrically connected through the wirings 10 to the leads 5b. The wirings 10 are flexible and extend with a slack so as to allow, in a later process, the stages 6 and 7 with the sensor chips 2 and 3 to be sloped or tilted from the plane including the frame 5. Each of the wirings 10 has a first bonding portion 10a that is bonded with the sensor chip 2 or 3 and a second bonding portion 10b that is bonded with the lead 5b. Tilting the stages 6 and 7 with the sensor chips 2 and 3 increases a distance between the first and second bonding portions 10a and 10b of the wiring 10. The slacks of the wirings 10 allow the stages 6 and 7 to be sloped or tilted.
As shown in FIG. 27B, the lead frame 4 is interposed between first and second dies “D” and “E”. The first die “D” has a concave portion and a ridge portion, while the second die “E” has a flat surface “E1”. The second die “E” moves toward the first die “D”, so that the flat surface “E1” presses up the projecting parts 8 and 9 until the first and second dies “D” and “E” sandwich the square frame portion 5a of the frame 5, whereby the twistable portions 5e are twisted, and the stages 6 and 7 are sloped or tilted from the plane that includes the frame 5.
As shown in FIG. 27C, the stages 6 and 7 are sloped or tilted from the plane that includes the frame 5. The magnetic sensor chips 2 and 3 which are respectively mounted on the stages 6 and 7 are also sloped or tilted together with the stages 6 and 7. The sloped magnetic sensor chips 2 and 3 have a predetermined slope angle with reference to the square frame portion 5a and to the flat surface “E1”. The predetermined slope angle is determined by the projecting parts 8 and 9. For example, the predetermined slope angle is determined by a distance between the twistable portion 5e and the top 8a or 9a of the projecting part 8 or 9.
A molten resin is injected into the cavity of the dies “D” and “E” while keeping the second die “E” to push up the projecting parts 8 and 9, thereby forming a resin mold 11 that encapsulates and fixes the sloped sensor chips 2 and 3 and the sloped stages 6 and 7. The rectangle frame portion 5a and the outside portions of the leads 5b and the modified connection leads 5d are positioned outside the resin mold 11. The second surfaces of the leads 5b and the modified connection leads 5d, the rectangle frame portion 5a, the outside portions of the leads 5b and the modified connection leads 5d are exposed from the resin mold 11.
The lead frame 4 with the resin mold 11 is then immersed into an electrolytic plating solution to form plating layers on the exposed surfaces. The leads 5b and the modified connection leads 5d are detruncated to remove the rectangle frame portion 5a and the outside portions of the leads 5b and the modified connection leads 5d, while leaving the plating layers on the second surfaces of the remaining portions of the leads 5b and the remaining portions of the modified connection leads 5d, thereby completing the sensor 1.
The sensor 1 is mounted on a circuit board so that the plating layers on the second surfaces of the remaining leads 5b and the remaining modified connection leads 5d are electrically connected with the circuit board.
FIG. 28 is a bottom plan view of a sensor, taken along an A-A line of FIG. 24. The injection molding process is performed while the top portions 8a and 9a of the projecting parts 8 and 9 are in contact with the flat surface “E1” of the die “E”. Thus, the molten resin when injected can not cover the top portions 8a and 9a of the projecting parts 8 and 9 so that the top portions 8a and 9a are exposed from the second surface 11a of the resin mold 11. When the sensor 1 is mounted on the circuit board, the exposed top portions 8a and 9a can electrically contact with conductive patterns of the circuit board, and a short circuit can be formed.
FIG. 29 is a cross sectional elevation view of a conventional lead frame interposed between dies “D” and “E”. As shown in FIG. 29, paired dies “D” and “E” are used. The die “D” has a concave “D1” and a ridge portion that are covered with a sheet “S”, while the die “E” has a flat surface “E1” that is covered with a sheet “S”. The sheet “S” is flexible. When the dies “D” and “E” are closed, then the top portions 8a and 9a come tightly into contact with the sheet “S”. In the injection molding process, the molten resin fills up the cavity without sealing or coating the top portions 8a and 9a, whereby the top portions 8a and 9a are not covered by the resin mold 11 and project from the bottom surface 11a of the resin mold 11. The sensor 1 has a bottom surface that includes a flat bottom of the resin mold 11, bottom surfaces of the leads 5b, and the projecting top portions 8a and 9a. The bottom surfaces of the leads 5b are leveled to the flat bottom of the resin mold 11. The projecting top portions 8a and 9a are different in level from the bottom surfaces of the leads 5b and the flat bottom of the resin mold 11. The projecting top portions 8a and 9a prevent the leads 5b from contacting with conductive patterns of the circuit board, whereby the leads 5b can not be electrically connected to the circuit board. The projecting top portions 8a and 9a further prevent the sensor from being satisfactorily mounted on the circuit board.
After the sensor with the resin mold 11 is released from the dies “D” and “E”, the sensor may optionally be immersed in an electrolytic plating solution, whereby a plating layer such as a solder layer is formed on not only the exposed bottom surfaces of the leads 5b but also the projecting top portions 8a and 9b. The plating layer that covers the projecting top portions 8a and 9b is unnecessary. When the sensor is mounted on the circuit board, the unnecessary plating layer covering the top portions 8a and 9a is interposed between the sensor and the circuit board. The unnecessary plating layer prevents the leads 5b from contacting with conductive patterns of the circuit board, whereby the leads 5b can not be electrically connected to the circuit board. The unnecessary plating layer further prevents the sensor from being satisfactorily mounted on the circuit board.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved lead frame, a sensor including the improved lead frame, a method of forming a lead frame and a method of forming a sensor using the lead frame. This invention addresses these needs in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.